FIELD OF THE INVENTION
The invention relates to a plate-type catalytic converter through which a flow medium flows from an inflow side to an outflow side and which includes a stack of first plates and second plates each having a catalytically active surface.
Such a plate-type catalytic converter is used, inter alia, for reducing the content of nitrogen oxides in the flue gas or waste gas of a combustion plant. The selective catalytic reduction (SCR) process is used in that case to convert nitrogen oxides together with a reducing agent introduced into the gas mixture in advance, generally ammonia NH.sub.3, into water and molecular nitrogen on a catalytically active surface. Instead of ammonia, the use of urea, which is decomposed into ammonia in the flue gas, is also known. For that purpose, the catalytically active material, with which the plates are coated on both sides, is generally formed of titanium dioxide TiO.sub.2 and one or more of the additives tungsten trioxide WO.sub.3, molybdenum trioxide MoO.sub.3 and vanadium pentoxide V.sub.2 O.sub.5.
A plate-type catalytic converter is composed of a stack of plates provided with a catalytically active surface and a holding device for the plates. The holding device which is used is generally an element casing in which the plates are evenly spaced and oriented parallel to one another. The element casing usually has a cuboid shape which is open at the ends, allowing a flow medium, for example a flue gas, to flow through the cuboid from the inflow side to the outflow side parallel to the long sides. The plate planes are aligned parallel to the flow direction. A plate-type catalytic converter of that type is disclosed in International Application No. WO 94/26411 A1, corresponding to U.S. Pat. No. 5,820,832.
A plurality of those element casings fitted with catalytically active plates are disposed alongside one another to form a catalyst module. A complete flue-gas cleaning unit in turn includes a number of banks of such catalyst modules disposed one after the other in a flue-gas duct. For example, a DeNOx unit for reducing the content of nitrogen oxides in the flue gas from a fossil fuel-fired power station usually has three to five banks of such catalyst modules.
In order to space the individual plates in an element casing, each plate is usually provided with a bead-like structure running continuously along the plate. In that way, a plurality of separate sub-chambers through which a flow medium flows from the inflow side to the outflow side are created between two directly adjacent plates. There is essentially a laminar flow of the flow medium through those sub-chambers, producing a disadvantageous flow profile for the catalytic conversion of the reactants, for example the nitrogen oxides and the ammonia. The flow medium flowing in the center of a sub-chamber only reaches the catalytically active surface of the sub-chamber through the mechanism of diffusion and not through turbulence. That generally hinders the contact of the reactants with the catalytically active surface of the sub-chambers which is necessary for the catalytic reaction.
International Application No. WO 94/26411 A1, corresponding to U.S. Pat. No. 5,820,832, describes a plate-type catalytic converter which is fitted with first plates, that have virtually linear elevations, beads or similar structures, and in addition is fitted with second plates. The structures of the first plates are aligned virtually parallel to the flow direction, while the structures of the second plates are transverse or inclined to the flow direction. In that way, turbulence can be generated within the sub-chambers between the individual plates. However, that type of catalytic converter has a disadvantage which is that the structures of the second plates cause a high pressure loss in the flow of the flow medium, and that soot and ash particles present in the flow medium can therefore easily settle at some points in the sub-chambers due to its low flow rate. A catalytic converter of that type therefore tends to become blocked.